The present invention deals with the detection of very small contour variations on the surface of an article and is particularly characterized by the ability to inspect curved surfaces, although it may be used on flat surfaces as well. The invention is not limited to any particular type of article being inspected, but has particular applicability to the detection of small contour variations that are exhibited as reflected optical distortion in bent glass sheets used for windows in vehicles such as automobiles and aircraft. It should be apparent that articles of other materials such as plastics, metal, or ceramic could be inspected for contour variations by the method and apparatus of the present invention. An example of the type of surface irregularity that the present invention is intended to detect is the waviness sometimes present that is observed in images reflected from the exterior of automotive windows.
Prior to the present invention the technique commonly used to inspect automotive glass for reflected optics has required the use of subjective human observation of the amount of distortion in a piece of glass mounted in front of a reference pattern, usually a series of black and white lines. The glass sample is mounted on edge on a rotatable table approximately midway between the reference pattern and the inspector. The inspector observes the reflection of the pattern from the glass surface by viewing the image at an angle of about 5 degrees between the line of sight and the plane of the surface. This method of quality inspection has several drawbacks and some inherent errors that cause considerable variability in the results. A variation in the angle of view from the fixed inspector's location from one end of the part to the other inherently introduces error into such an inspection system. With a large part, the angle can vary on the order of 20%, which can introduce significant error. The variation in angle is even greater with a curved part. As a consequence, the angle variation causes the opposite ends of the part to be judged unequally even though the optical quality may in fact be uniform. Additionally, human inspectors judge the optical characteristics with a considerable degree of subjective variation. As a result, consistent evaluation of optical distortion quality was difficult by prior art techniques.
Optical power of lenses has been measured using gauges that contact the surface of a lens. These devices measure the curvature of the whole article and are not designed to detect the very small deviations in the curvature that are characteristic of optical defects.
In U.S. Pat. No. 3,470,739 (Takafuji et al.) a scanning device is employed to determine the flatness of a strip of sheet metal. The device employs two contact probes that move along the surface, the relative elevations of which, with respect to a reference plane, are used to calculate the slope of the surface. The arrangement of the patent would not be suitable for measuring the small amplitude irregularities on a surface that are observed as optical distortion, particularly when the part is intended to have an overall curvature. Mere substitution of higher sensitivity probes to detect small changes in surface contour is not feasible due to the signal to noise ratio relative to the magnitude of variations to be detected. Also, such an approach would not take into account the curvature of parts that are not flat by design. The above-referenced patent measures the total slope of the surface which, particularly for curved surfaces, would be superimposed on the optical distortion measurement, thus compounding the problem of signal to noise ratio and the sensitivity of the detecting device relative to its measuring range. The criticality of this difference is apparent when it is considered that a typical automotive transparency may have a depth of bend of several inches, whereas the amplitude of severe optical distortion may be on the order of 0.001 to 0.005 inch, and inspection systems of the type involved here would preferably be able to detect distortion having amplitudes on the order of about 0.0001 to 0.001 inch.
U.S. Pat. No. 3,135,055 (Butler et al.) discloses an inspection system in which a probe scans the surface of an article and the deflection of the probe is recorded to provide a digital mapping of the surface contour. Such an arrangement does not lend itself to measuring optical distortion on the surface of a curved article since the mapping would include the design curvature as well as the minor surface irregularities, and the order of magnitude difference between the two curvatures renders it extremely difficult to provide the degree of sensitivity required to detect the small amplitude distortion.
U.S. Pat. Nos. 3,311,233 (Helmbrecht), 3,733,704 (Farabaugh), 3,869,800 (Bartlett), and 4,221,053 (Bobel et al.) show contour check fixtures in which a plurality of probes are employed to determine deviations of the overall curvature or shape from the specified contour. These systems are not intended to detect small distortions on the surface, and their sensitivity is limited by the number of probes that can be provided in a given area.